Microscope tube lens, microscope apparatus and image pickup optical system

ABSTRACT

A tube lens for a microscope includes, in the order from an object, a first lens group that has positive power and that includes a cemented lens, a second lens group that has negative power, and a third lens group has positive power. The tube lens satisfies the following condition expressions, where NA is a numerical aperture on an image side of the tube lens, FN is a field number of the tube lens, and ε is an airy disk diameter with respect to d line of the tube lens (588 nm):
 
0.04&lt;NA; and
 
1700≦FN/ε.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2014-207624, filed Oct. 8, 2014,the entire contents of which are incorporated herein by this reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention is related to a microscope tube lens, a microscopeapparatus and an image pickup optical system.

Description of the Related Art

In recent years, accompanying the remarkable increase in the number ofpixels of an image pickup element, expectation has become higher formicroscope apparatuses that realize observation and image acquisitionwith a wide field of view and high resolution. For example, when such amicroscope apparatus is applied to the field of virtual slide, higherscanning speeds can be attained. Also, when such a microscope apparatusis applied to the fields of the brain research etc., observation is madepossible in which cells are studied in detail while viewing the entirebrain containing the cells in a real-time manner.

In order to realize such a microscope apparatus, an objective with awide field of view is employed, and thus tube lenses that cansufficiently bring out the performance of such objectives are desired. Atube lens is described in for example Japanese Laid-open PatentPublication No. 04-093911.

SUMMARY OF THE INVENTION

An aspect of the present invention provides a microscope tube lens thatforms an enlarged image of an object by being combined with aninfinity-corrected objective, the tube lens including in an order froman object side a first lens group that has positive power and thatincludes a cemented lens, a second lens group that has negative power,and a third lens group that consists of a plurality of lenses eachhaving positive power and that has positive power as a whole, wherein afollowing condition expression is satisfied, where FL is a focal lengthof the tube lens and D2 is a distance from a lens surface that isclosest to the object in the tube lens to a position of an entrancepupil of the tube lens:0.3<D2/FL<1.3.

Another aspect of the present invention provides a microscope apparatusincluding a microscope tube lens according to the aspect above.

Another aspect of the present invention provides an image pickup opticalsystem including an infinity-corrected objective and a microscope tubelens, wherein the microscope tube lens includes in an order from anobject side a first lens group that has positive power and that includesa cemented lens, a second lens group that has negative power, and athird lens group that consists of a plurality of lenses each havingpositive power and that has positive power as a whole, and a followingcondition expression is satisfied, where FL is a focal length of thetube lens and D2 is a distance from a lens surface that is closest to anobject in the tube lens to a position of an entrance pupil of the tubelens:0.3<D2/FL<1.3.

Another aspect of the present invention provides a microscope tube lensthat forms an enlarged image of an object by being combined with aninfinity-corrected objective, the tube lens including in an order froman object side a first lens group that has positive power and thatincludes a cemented lens, a second lens group that has negative power,and a third lens group that has positive power, wherein a followingcondition expression is satisfied, where NA is a numerical aperture onan image side of the tube lens, FN is a field number of the tube lens,and ε is an airy disk diameter with respect to d line (588 nm) of thetube lens:0.04<NA; and1700≦FN/ε.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more apparent from the following detaileddescription when the accompanying drawings are referenced.

FIG. 1 exemplifies a microscope apparatus according to an embodiment ofthe present invention;

FIG. 2 is a sectional view of a tube lens according to example 1 of thepresent invention;

FIG. 3A through FIG. 3E are aberration diagrams of the tube lensexemplified in FIG. 2, and respectively show a spherical aberration, achromatic aberration of magnification, astigmatism, a comma aberrationand a distortion aberration;

FIG. 4 is a sectional view of a tube lens according to example 2 of thepresent invention;

FIG. 5A through FIG. 5E are aberration diagrams of the tube lensexemplified in FIG. 4, and respectively show a spherical aberration, achromatic aberration of magnification, astigmatism, a comma aberrationand a distortion aberration;

FIG. 6 is a sectional view of a tube lens according to example 3 of thepresent invention;

FIG. 7A through FIG. 7E are aberration diagrams of the tube lensexemplified in FIG. 6, and respectively show a spherical aberration, achromatic aberration of magnification, astigmatism, a comma aberrationand a distortion aberration;

FIG. 8 is a sectional view of a tube lens according to example 4 of thepresent invention;

FIG. 9A through FIG. 9E are aberration diagrams of the tube lensexemplified in FIG. 8, and respectively show a spherical aberration, achromatic aberration of magnification, astigmatism, a comma aberrationand a distortion aberration;

FIG. 10 is a sectional view of a tube lens according to example 5 of thepresent invention;

FIG. 11A through FIG. 11E are aberration diagrams of the tube lensexemplified in FIG. 10, and respectively show a spherical aberration, achromatic aberration of magnification, astigmatism, a comma aberrationand a distortion aberration;

FIG. 12 is a sectional view of a tube lens according to example 6 of thepresent invention;

FIG. 13A through FIG. 13E are aberration diagrams of the tube lensexemplified in FIG. 12, and respectively show a spherical aberration, achromatic aberration of magnification, astigmatism, a comma aberrationand a distortion aberration;

FIG. 14 is a sectional view of a tube lens according to example 7 of thepresent invention;

FIG. 15A through FIG. 15E are aberration diagrams of the tube lensexemplified in FIG. 14, and respectively show a spherical aberration, achromatic aberration of magnification, astigmatism, a comma aberrationand a distortion aberration;

FIG. 16 is a sectional view of a tube lens according to example 8 of thepresent invention;

FIG. 17A through FIG. 17E are aberration diagrams of the tube lensexemplified in FIG. 16, and respectively show a spherical aberration, achromatic aberration of magnification, astigmatism, a comma aberrationand a distortion aberration;

FIG. 18 is a sectional view of a tube lens according to example 9 of thepresent invention; and

FIG. 19A through FIG. 19E are aberration diagrams of the tube lensexemplified in FIG. 18, and respectively show a spherical aberration, achromatic aberration of magnification, astigmatism, a comma aberrationand a distortion aberration.

DESCRIPTION OF THE EMBODIMENTS

The tube lens described in Japanese Laid-open Patent Publication No.04-093911 does not provide sufficient correction of field curvature orcomma aberrations.

Accordingly, it is difficult to realize high resolution in the peripheryportions of the field of view in a microscope apparatus having the tubelens. Also, higher numerical apertures tend to make more considerablethe deterioration of performance caused by the off-axis aberration. Thismakes it difficult to realize both a wide field of view and highresolution.

In view of the above, the embodiments of the present invention will beexplained.

FIG. 1 is a schematic view showing a microscope apparatus 100 accordingto an embodiment of the present invention. The microscope apparatus 100exemplified in FIG. 1 includes a microscope body 2, a light sourcedevice 3 connected to the microscope body 2 via an optical fiber 4, animage pickup optical system having an objective 8 and a tube lens 1, anda digital camera including an image pickup element 9. The microscopeapparatus 100 further includes an X-Y handle 6 for moving a stage 5 inthe directions orthogonal to the optical axis of the objective 8, and aZ handle 7 for moving the stage 5 in the directions parallel to theoptical axis of the objective 8. Also, the microscope apparatus 100 mayinclude an eyepiece, which is omitted in the drawings.

The objective 8 is an infinity-corrected microscope objective. Theobjective 8 is configured to have a wide field of view (i.e., lowmagnification and a high field number) and a high numerical aperture.

The tube lens 1 is a tube lens for a microscope (referred to as amicroscope tube lens) that forms an enlarged image of an object (sampleS) by being combined with the objective 8. The tube lens 1 is configuredto provide god correction of aberrations and to have a wide field ofview and a high numerical aperture.

The image pickup element 9 is for example a Charge Coupled Device (CCD),a Complementary Metal Oxide Semiconductor (CMOS), etc., and is arrangedon an imaging plane on which an enlarged image is formed by theobjective 8 and the tube lens 1.

It is desirable that the size of the image pickup element 9 be large sothat the high field numbers of the objective 8 and the tube lens 1 areutilized sufficiently. It is also desirable that the image pickupelement 9 be of high definition so that the high imaging performance ofthe objective 8 and the tube lens 1 are utilized sufficiently. It isdesirable for example that the pixel size L (i.e., the length of oneside of each pixel) of the image pickup element 9 be from 1μ through 1μ.One reason for this is that when the pixel size L is greater than 17μ,the Nyquist frequency becomes lower than the cutoff frequency of theimage pickup optical system, leading to a situation where the resolutionperformance of the image pickup optical system is not utilizedsufficiently. Another reason is that when the pixel size L is smallerthan 1μ, the Nyquist frequency becomes too higher than the cutofffrequency of the image pickup optical system, leading to a situationwhere the resolution performance of the image pickup element is notutilized sufficiently.

In the microscope apparatus 100, sample S disposed on the stage 5 isilluminated by light that was emitted from the light source device 3 andhas been incident via the optical fiber 4. Illuminated sample S isprojected on the image pickup element 9 in an enlarged state by theobjective 8 and the tube lens 1, and the enlarged image of sample Sformed by the objective 8 and the tube lens 1 is picked up by the imagepickup element 9. When the microscope apparatus 100 has an eyepiece, theenlarged image of sample S is observed by that eyepiece.

The microscope apparatus 100 having the above configuration makes itpossible to conduct observation and image acquisition with wide field ofview and high resolution.

Next, explanations will be given for the configuration and effects ofthe tube lens 1.

As described above, the tube lens 1 is a microscope tube lens that formsan enlarged image of an object (sample S) by being combined with theobjective 8, and condenses the infinite light beam (also referred to asan infinite light flux) from the objective 8 into the imaging plane onwhich the image pickup element 9 is arranged. The tube lens 1 includes,in the order from the object side, a first lens group that has positivepower and that includes a cemented lens, a second lens group havingnegative power and a third lens group that has positive power as a wholeand that consists of a plurality of lenses, each having positive power.

The first lens group mainly has a function of converting a parallellight beam arriving from the objective into a convergent light beam soas to decrease the height of an off-axis ray by using the positivepower, and a function of correcting the spherical aberration and theaxial chromatic aberration by using the cemented lens included in thefirst lens group.

The second lens group uses the negative power so as to lower theconvergence degree by refracting the convergent light beam from thefirst lens group in the diffusion direction, and thereby outputs a raywhile increasing the height of the ray toward the third lens group.

The third lens group mainly has a function of using the plurality ofpositive lenses to achieve a balance between the axial aberration andthe off-axis aberration by correcting the chromatic aberration ofmagnification and the distortion aberration that occurred in the firstand second lens groups, and a function of using the positive power thatthe third lens group has as a whole to condense the ray onto the imagingplane. Note that the off-axis principal ray is the highest in the thirdlens group.

The tube lens 1 employs a configuration of a positive lens group, anegative lens group and a positive lens group in this order so that thesecond lens group has negative power, and thereby can provide goodcorrection of the spherical aberration, the astigmatism, the commaaberration and the Petzval sum. Also, this configuration can maintainhigh telecentricity on the image side even when the distance is longbetween the position of the exit pupil of the objective and the tubelens 1. Accordingly, it is possible to provide good correction of theaxial aberration and the off-axis aberration. Further, the fieldcurvature and the astigmatism are also corrected, making it possible toform, on the imaging plane, an image that is homogeneous from the centerto the peripheries and that is based on a wide field of view.

Further, because comma aberrations tend to occur more in the third lensgroup, in which the height of an off-axis ray is high, than in the firstor second lens groups, it is very important in view of good correctionof the comma aberration in the entire tube lens 1 to suppress the amountof generated comma aberrations in the third lens group. Regarding thispoint, by including a plurality of lenses having positive power in thethird lens group, it is possible for the tube lens 1 to make relativelylarge a curvature radius of each of such lenses having positive powerwhile realizing prescribed positive power in the third lens group as awhole. Thereby, it is possible to prevent the angle of an off-axismarginal ray incident on a lens surface and the refraction angle at thelens from becoming too large. As a result of this, because the amount ofgenerated comma aberrations in the third lens group is suppressedsufficiently, it is possible to provide good correction of commaaberrations in the tube lens 1.

Hereinafter, a configuration desirable for the tube lens 1 will beexplained.

It is desirable that the first lens group include a lens having theconcave surface facing toward the image side (referred to as a firstlens hereinafter). By employing a configuration in which the tube lens 1includes the first lens in the first lens group, when the convergentlight beam obtained through the conversion, conducted by the first lensgroup, from the parallel light beam is incident on the concave surfaceof the first lens facing toward the image side, the incident angle ofthe convergent light beam becomes smaller. Accordingly, the negativepower of this concave surface has effects of suppressing the sphericalaberration, the comma aberration and the astigmatism occurring in thefirst lens group. Also, the negative power of this concave surface alsocontributes to the reduction in the Petzval sum.

Further, it is desirable that the first lens included in the first lensgroup be a meniscus lens including a single lens or a cemented lenshaving the concave surface facing toward the image side, and it is moredesirable that the first lens be arranged closest to the image in thefirst lens group. In such a case, the parallel light beam from theobjective 8 is converted into a convergent light beam before beingincident on the first lens in the first lens group, and is incident onthe first lens.

When the first lens is a meniscus lens, the surface facing toward theobject side is convex, making the angle of light incident on thatsurface smaller. Thereby it is possible to reduce the sphericalaberration, the comma aberration and the astigmatism occurring on theconvex surface of the first lens. Also, it is possible to distribute thepositive power of the first lens group to lenses or a lens group, whichis closer to the object than the first lens is (referred to as a first-alens group hereinafter) and to the convex surface of the first lens.This makes it possible to moderate the curve of a ray on each surface inthe first lens group. Thereby, it is possible to suppress the amount ofthe spherical aberrations occurring in the first lens group.Configurations including such a first lens are advantageous inpreventing the deterioration of the peripheral performance and cancontribute to better correction of comma aberrations in a tube lenshaving an entrance pupil with a large diameter and having a high fieldnumber such as the tube lens 1.

It is desirable that the second lens group include a lens having theconcave surface facing toward the object side (referred to as a secondlens hereinafter). By employing a configuration in which the tube lens 1includes the second lens in the second lens group, when a convergentlight beam output from the first lens group to be incident on the secondlens group is incident on the concave surface of the second lens facingtoward the object side, the incident angle of the convergent light beambecomes larger. Accordingly, the negative power of this concave surfacehas large effects on the convergent light beam, causing an aberration inthe direction to cancel the spherical aberration, the comma aberrationand the astigmatism caused in the first lens group.

Further, it is desirable that the second lens included in the secondlens group be a meniscus lens including a single lens or a cemented lenshaving the concave surface facing toward the object side, and it is moredesirable that the second lens be arranged closest to the object in thesecond lens group. As described above, the second lens group has afunction of increasing the height of the off-axis principal ray towardthe third lens group. When the surface of the second lens facing towardthe image side is flat or concave, the height of the ray output from thesecond lens increases sharply. This leads to a too large angle of theoff-axis principal ray incident on the third lens group, increasing thegeneration amount of the off-axis aberrations (comma aberrations andchromatic aberrations of magnification) in the third lens group.Therefore, it is desirable that the second lens be a meniscus lenshaving the concave surface facing toward the object side in order tosuppress the occurrence of off-axis aberrations in the third lens group.It is also possible to configure the second lens group by using onlymeniscus lens. Configurations including such a second lens areadvantageous in preventing the deterioration of the peripheralperformance and can contribute to better correction of the commaaberration and the chromatic aberration of magnification in a tube lenshaving an entrance pupil with a large diameter and having a high fieldnumber such as the tube lens 1.

Accordingly, a configuration in which the first lens group includes thefirst lens and the second lens group includes the second lens isdesirable in that it is possible to suppress the amounts of sphericalaberrations, comma aberrations and astigmatism that have been caused inthe light output from the second lens group via the first lens group andthat it is possible to correct better the field curvature.

The first and second lenses have a function of correcting, by using theconcave surfaces having negative power, chromatic aberrations by causingan axial chromatic aberration and a chromatic aberration ofmagnification in the direction opposite to the axial chromaticaberration and the chromatic aberration of magnification that haveoccurred in the first-a lens group having positive power. Accordingly,in order to cancel chromatic aberrations efficiently, it is desirablethat the first lens and second lens include at least four lenses intotal with at least one lens being a cemented lens. It is more desirablethat both of the first and second lenses be cemented lenses.Configurations including such first and second lenses are advantageousin preventing the deterioration of the peripheral performance and cancontribute to better correction of the chromatic aberration ofmagnification in a tube lens having an entrance pupil with a largediameter and having a high field number such as the tube lens 1.

Note that also in a case where a cemented lens is placed in otherlocations such as the third lens group etc., it is possible to achieve abalance of chromatic aberrations by reducing chromatic aberrationscaused from positive power. In such a case, however, because the heightsof the off-axis principal ray and the marginal ray are high, it isdifficult to conduct a fine adjustment of chromatic aberrations. Thismakes it necessary to prepare many lenses for correction of aberrations,increasing the number of lenses constituting the tube lens 1.

It is desirable that the third lens group include a meniscus lens withpositive power having the concave surface facing toward the object side.This makes it possible to suppress the angle at which the off-axisprincipal ray is incident on that meniscus lens with its height beingincreased while travelling from the second lens group. Thereby, it ispossible to provide good correction of comma aberrations whilecorrecting astigmatism.

Hereinafter, conditions satisfied by the tube lens 1 will be explained.

The tube lens 1 is configured to satisfy following condition expression(4).0.3<D2/FL<1.3  (4)

In the above expression, FL is the focal length of the tube lens 1, andD2 is the distance between the lens surface closest to the object intube lens 1 (the lens surface closest to the objective 8) and theposition of the entrance pupil of the tube lens 1 (the position of theexit pupil of the objective 8). Note that a tube lens of a microscope isusually arranged in such a manner that the position of the entrancepupil of the tube lens and the position of the exit pupil of theobjective coincide. Accordingly, explanations below is based on anassumption that the position of the entrance pupil of a tube lens andthe position of the exit pupil of an objective refer to substantiallythe same position.

Condition expression (4) represents a condition for good correction ofthe comma aberration and the astigmatism of an off-axis ray and theaxial spherical aberration in a case when the position of the entrancepupil of the tube lens 1 (the position of the exit pupil of theobjective 8) is on the object side of the tube lens 1. Also, bysatisfying condition expression (4), the tube lens 1 can realize hightelecentricity. This makes it to possible to cause a ray guided from thetube lens 1 to be incident on the image pickup element such as a CCDimage sensor etc. in a state advantageous for the image pickup element(i.e., state in which the ray is almost parallel to the optical axis).By satisfying at least condition expression (4), the tube lens 1 canrealize excellent optical performance compatible with an objective witha wide field of view.

When D2/FL has exceeded the upper limit value in condition expression(4), the position of the entrance pupil of the tube lens 1 (the positionof the exit pupil of the objective) becomes too distant from the firstlens group. This extremely increases the height of an off-axis ray whenit is incident on the first lens group, increasing the comma aberrationand the astigmatism of the off-axis ray and the spherical aberration.This also decreases the telecentricity on image side. Further, becausethe outer diameter of the tube lens 1 also becomes larger, decreasingthe productivity. Meanwhile, when D2/FL has fallen below the lower limitvalue, the position of the entrance pupil of the tube lens 1 (positionof the exit pupil the objective) becomes too close to the first lensgroup. This prevents the height of the off-axis ray from becoming highsufficiently when the ray is incident on the first lens group. Thismakes it difficult to provide good correction of the off-axis commaaberration and the astigmatism. This also decreases the telecentricityon the image side.

Hereinbelow, explanations will be given for conditions that it isdesirable that the tube lens 1 satisfy.

It is desirable that the tube lens 1 satisfy the following conditionexpressions.0.3<FLG1/FL<3  (5)−4<FLG2/FL<−0.05  (6)0.3<D1/D0<0.8  (7)

In the above expressions, FLG1 is the focal length of the first lensgroup and FLG2 is the focal length of the second lens group. Also, D0 isthe distance between the lens surface closest to the object in the tubelens 1 and the imaging plane on which the enlarged image is formed, andD1 is the distance between the lens surface closest to the object in thetube lens 1 and the lens surface closest to the image in the tube lens1.

Condition expression (5) defines the relationship between the focallength of the first lens group and the entire focal length. Conditionexpression (6) defines the relationship between the focal length of thesecond lens group and the entire focal length. By satisfying conditionexpressions (5) and (6), the power distribution between the first lensgroup and the second lens group of the tube lens 1 becomes appropriate.Thereby, the tube lens 1 provides better correction of the sphericalaberration and the comma aberration in the entirety of the tube lens 1and can also provide better correction of field curvature by reducingthe Petzval Sum by using the negative power of the second lens group.Particularly, better correction of the field curvature and the commaaberration is important in preventing the deterioration of theperipheral performance in a tube lens having an entrance pupil with alarge diameter and having a high field number such as the tube lens 1.Therefore, it is desirable that condition expressions (5) and (6) besatisfied.

When FLG1/FL has exceeded the upper limit value in condition expression(5), the power of the first lens group becomes too low relative to thepower of the entire tube lens 1. Accordingly, the power of other lensgroups including the second lens group similarly becomes too lowrelative to the power of the entire tube lens. This increases thePetzval Sum, worsening the field curvature and the comma aberration.Meanwhile, when FLG1/FL has fallen below the lower limit value, thepower of the first lens group becomes too high relative to the power ofthe entire tube lens 1. Thereby, the power of other lens groupssimilarly becomes high relative to the power of the entire tube lens,worsening the spherical aberration and the comma aberration. Also,because the power of each lens group is high and the eccentricitysensitivity is high, slight lens eccentricity worsens various types ofaberration.

When FLG2/FL has exceeded the upper limit value in condition expression(6), the power of the second lens group becomes too high relative to thepower of the entire tube lens 1. Accordingly, the power of other lensgroups similarly becomes too high relative to the power of the entiretube lens, worsening the spherical aberration and the comma aberration.Also, because the power of each lens group is high and the eccentricitysensitivity is high, slight lens eccentricity worsens various types ofaberration. Meanwhile, when FLG2/FL has fallen below the lower limitvalue, the power of the second lens group becomes too low relative tothe power of the entire tube lens 1. This increases the Petzval Sum,worsening the field curvature and the comma aberration.

Condition expression (7) defines the relationship between the distancefrom the lens surface closest to the object in the tube lens 1 (referredto as a first surface hereinafter) to the imaging plane, which is theimaging position, and the entire length of the tube lens 1, whichcorresponds to the distance from the first surface to the lens surfaceclosest to the image in the tube lens 1 (referred to as a last surfacehereinafter). The lens surface closest to the object in the tube lens 1is the lens surface closest to the objective 8 in the tube lens 1. Thelens surface closest to the image in the tube lens 1 is the lens surfaceclosest to the imaging plane. By satisfying condition expression (7),the tube lens 1 can provide good correction of the spherical aberration,the comma aberration and the astigmatism on the imaging plane andrealize high telecentricity on the image side without extremelyelongating the entire length of itself. Particularly, better correctionof the field curvature and the comma aberration is important inpreventing the deterioration of the peripheral performance in a tubelens having an entrance pupil with a large diameter and having a highfield number such as the tube lens 1. Therefore, it is desirable thatcondition expression (7) be satisfied.

When D1/D0 has exceeded the upper limit value in condition expression(7), the distance from the third lens group to the imaging positionbecomes too short. This makes it difficult to arrange an image pickupelement, an optical path splitting element, a parfocality adjustmentmechanism, etc., which are to be arranged on the image side of the tubelens 1. When D1/D0 has fallen below the lower limit value, the distancefrom the first lens group to the third lens group becomes too short,making it difficult to correct spherical aberrations and commaaberrations. Also, even when spherical aberrations and comma aberrationshave been corrected, because the power of each lens group becomes toohigh, the eccentricity sensitivity of lens groups increases, leading toa situation where slight lens eccentricity worsens various types ofaberrations.

It is desirable that the tube lens 1 satisfy the following conditionexpressions when the first lens group includes, at the position closestto the image, the first lens that is a meniscus lens having the concavesurface facing toward the image side and the second lens group includes,at the position closest to the object, the second lens that is ameniscus lens having the concave surface facing toward the object side.30<ν1−νs  (8)30<ν11−νs1  (9)25<ν12−νs2  (10)

Note, when at least one of the first and second lenses is a cementedlens including a lens with positive power and a lens with negative power(i.e., an achromatizing lens), ν1 is the highest Abbe number among Abbenumbers of lenses with positive power included in that cemented lens.Also, νs is the smallest Abbe number among Abbe numbers of lenses withnegative power included in that cemented lens.

Also, when each of the first lens and the second lens is a cemented lensincluding a lens with positive power and a lens with negative power(i.e., an achromatizing lens), ν11 is the highest Abbe number among Abbenumbers of lenses with positive power included in a first cemented lens,which is one of the first and second lenses. νs1 is the smallest Abbenumber among Abbe numbers of lenses with negative power included in thefirst cemented lens. ν12 is the highest Abbe number among Abbe numbersof lenses with positive power included in a second cemented lens, whichis the other of the first and second lenses. νs2 is the smallest Abbenumber among Abbe numbers of lenses with negative power included in thesecond cemented lenses.

Condition expressions (8) through (10) define a difference of Abbenumbers of glass materials of achromatizing lenses (first lens and/orsecond lens). By satisfying condition expression (8), it is possible tocancel the chromatic aberration of magnification and the axial chromaticaberration that have occurred in the first lens group so as to providegood correction of the chromatic aberration as a whole. When ν1−νs hasfallen below the lower limit value in condition expression (8), theamount of correction of the chromatic aberration of magnification andthe axial chromatic aberration in an achromatizing lens becomes smaller,making it difficult to provide good correction of the chromaticaberration on the imaging plane. Note that achromatizing lenses arelenses that correct chromatic aberrations by causing chromaticaberrations in the direction opposite to the direction of chromaticaberrations that have occurred previously (i.e., on the object side).

When the imaging system is to be configured by using the objective 8that is of an infinity-corrected type and the tube lens 1, the objective8 with a high field number tends to have chromatic aberrations ofmagnification caused in a fixed direction, sometimes making it difficultto correct aberrations. In such a case, it is desirable to use the tubelens 1 for causing chromatic aberrations of magnification in thedirection opposite to the objective 8 in order to cancel the chromaticaberration of magnification with the entire imaging system. Bysatisfying condition expressions (9) and (10), it is possible toexcessively correct chromatic aberrations occurring in the tube lens 1by using a plurality of achromatizing lenses. This also makes itpossible to provide good correction of chromatic aberrations ofmagnification, which the objective 8 failed to correct completely, inaddition to the chromatic aberration of magnification and the axialchromatic aberration having occurred in the first lens group. Whenν11-νs1 has fallen below the lower limit value in condition expression(9) or when ν12−νs2 has fallen below the lower limit value in conditionexpression (10), the amount of correction of the chromatic aberration ofmagnification and the axial chromatic aberration becomes smaller. Thismakes it difficult to correct completely the chromatic aberration ofmagnification having occurred in the objective 8.

In other words, in order to attain high performance of correctingchromatic aberration by using a tube lens alone, it is desirable thatthe tube lens 1 satisfy condition expression (8), and in order to alsocorrect chromatic aberration that has not been corrected by theobjective 8 completely, it is desirable that condition expressions (9)and (10) be satisfied.

When the first lens group includes the first lens having the concavesurface facing toward the image side and the second lens group includesthe second lens having the concave surface facing toward the objectside, it is desirable that the tube lens 1 satisfy the followingcondition expressions.0<|RG2/RG1|<3  (11)1.5<NdG2  (12)70<νdG1  (13)

In the above expressions, RG1 is a curvature radius of the concavesurface of the first lens facing toward the image side and RG2 is acurvature radius of the concave surface of the second lens facing towardthe object side. NdG2 is a refractive index with respect to d line ofthe second lens or, in a case when the second lens is a cemented lens, arefractive index with respect to d line of the lens closest to theobject among lenses constituting the second lens. νdG1 is the highestAbbe number among Abbe numbers of lenses with positive power included inthe first lens group. Note that when the first lens group includes aplurality of concave surfaces facing toward the image side, it isdesirable that RG1 be the curvature radius of the concave surfaceclosest to the image.

Condition expression (11) defines the ratio of the curvature radius ofthe concave surface of the second lens facing the object to thecurvature radius of the concave surface of the first lens facing towardthe image side. By satisfying condition expression (11), these lenseseffectively contribute to the suppression of aberration, making itpossible to suppress the amount of various types of aberrations causedin light output from the second lens group via the first lens group.

When |RG2/RG1| has exceeded the upper limit value in conditionexpression (11), the curvature radius of the concave surface of thefirst lens facing toward the image side becomes too small or thecurvature radius of the concave surface of the second lens facing towardthe object side becomes too large. When the curvature radius of theconcave surface facing toward the image side has become too small, thenegative power generated on the concave surface of the first lens becometoo high, worsening the spherical aberration, the comma aberration, andthe astigmatism. Also, when the curvature radius of the concave surfaceof the second lens has become too large, the negative power of thesecond lens group becomes too low, worsening the field curvature and thecomma aberration.

Condition expression (12) defines a refractive index with respect to dline of the second lens. By satisfying condition expression (12), it ispossible to provide good correction of the field curvature bysuppressing the Petzval Sum. This makes it possible to reduce thegeneration amount of the spherical aberration, the astigmatism and thecomma aberration in each lens group. When NdG2 has fallen below thelower limit value in condition expression (12), in order to generate therequired power in the second lens, it is necessary to considerablyreduce the curvature radius of the concave surface of the second lensfacing toward the object so that. This makes it difficult to providegood correction of the spherical aberration, the comma aberration andthe field curvature as the whole of the tube lens 1.

Condition expression (13) defines the highest Abbe number among Abbenumbers of lenses with positive power included in the first lens group.By satisfying condition expression (13), it is possible to provide goodcorrection of the spherical aberration and the axial chromaticaberration in the first lens group, in which the height of the axial rayis the highest. When νdG1 has fallen below the lower limit value incondition expression (13), it is difficult to provide good correction ofthe spherical aberration and the axial chromatic aberration.

When the third lens group includes the third lens with the positivepower at the position closest to the object and the fourth lens with thepositive power at the position closest to the image, it is desirablethat the tube lens 1 satisfy the following condition expression, whereFLG3a is the focal length of the third lens, and FLG3b is the focallength of the fourth lens. Note that the third and fourth lenses may besingle lenses or cemented lenses.0.2<FLG3b/FLG3a<5  (14)

Condition expression (14) defines the power distribution in the thirdlens group. By satisfying condition expression (14), the positive powerof the third lens group is distributed appropriately, making it possibleto provide good correction of the spherical aberration, the commaaberration and the field curvature. When FLG3b/FLG3a has fallen belowthe lower limit value in condition expression (14), the power of thefourth lens becomes too high relative to the power of the third lens.This cases large comma aberrations and field curvature in the fourthlens, in which the height of the off-axis principal ray is high. WhenFLG3b/FLG3a has exceeded the upper limit value in condition expression(14), the power of the third lens becomes too high relative to the powerof the fourth lens. This causes a large spherical aberration in thethird lens group, increasing the amount of aberrations occurring in thetube lens 1 as a whole. Configurations satisfying condition expression(14) are advantageous in preventing the deterioration of the peripheralperformance and can contribute to better correction of comma aberrationsand chromatic aberrations of magnification in a tube lens having anentrance pupil with a large diameter and having a high field number suchas the tube lens 1.

It is desirable that the tube lens 1 satisfy the following conditionexpressions.1.7<NdG3  (15)νdG3<45  (16)

In the above expression, NdG3 is the highest refractive index among therefractive indexes of lenses included in the third lens group and νdG3is the lowest Abbe number among Abbe numbers of lenses included in thethird lens group.

Condition expression (15) defines the refractive index with respect to dline of a positive lens included in the third lens group. By satisfyingcondition expression (15), it is possible to reduce, by using a lenswith the positive power of the third lens group, the distortionaberration caused by the negative power of the second lens group whileproviding good correction of the off-axis comma aberration. When NdG3has fallen below the lower limit value, the curvature radius of the lenssurface of a positive lens included in the third lens group becomessmaller, worsening the comma aberration or the distortion aberration.

Condition expression (16) defines the Abbe number of a positive lensincluded in the third lens group. By satisfying condition expression(16), it is possible to provide good correction of the off-axischromatic aberration of magnification in the third lens group, in whichthe off-axis principal ray become the highest. When νdG3 has exceededthe upper limit value, it is difficult to provide good correction of thechromatic aberration of magnification.

It is desirable that the tube lens 1 satisfy the following conditionexpressions.0.3<Ha/Hi<1  (17)−0.04<(Ea−Ec)/FL<0.03  (18)

In the above expressions, Ha is the height of the off-axis principal raywhen it is incident on the lens surface closest to the object in thetube lens 1, and Hi is the height of that off-axis principal ray when itis incident on the imaging plane. Ea is the maximum effective diameterof a lens included in the first lens group. Ec is the maximum effectivediameter of a lens included in the third lens group.

Condition expression (17) defines the height of the off-axis principalray. By satisfying the condition expression (17), the height of theoff-axis principal ray is maintained within an appropriate range, makingit possible to provide good correction of the comma aberration of thetube lens 1. When Ha/Hi has fallen below the lower limit value, thecurve of the lower marginal ray of the off-axis light beam becomes toomoderate, leading to insufficient correction of the comma aberration inthe third lens group. Meanwhile, when Ha/Hi has exceeded the upper limitvalue, the curve of the upper marginal ray of the off-axis light beambecomes too sharp, leading to a situation where the comma aberrationbecomes abruptly worsens on the off-axis. Configurations satisfyingcondition expression (17) are advantageous in preventing thedeterioration of the peripheral performance and can contribute to bettercorrection of comma aberrations and chromatic aberrations ofmagnification in a tube lens having an entrance pupil with a largediameter and having a high field number such as the tube lens 1.

Condition expression (18) defines the effective diameter of a lens. Bysatisfying condition expression (18), it is possible to reduce thedeterioration of the exit NA of an off-axis light beam with respect tothe exit NA of an on-axis light beam. When (Ea−Ec)/FL has fallen belowthe lower limit value, the diameter of the first lens group becomesmaller, worsening the deterioration of NA in case of an off-axis lightbeam. This causes differences in the brightness between a portion with ahigh image height and a portion with a low image height, resulting inunevenness in light amount. Meanwhile, when (Ea−Ec)/FL has exceeded theupper limit value, the diameter of the first lens group becomes toolarge, leading to a wider area through which the effective light beamdoes not pass. This also deteriorates the lens processing accuracy andincreases the degree of difficulty of processing frames.

It is desirable that the tube lens 1 be configured to satisfy thefollowing condition expression.0.04<NA  (1)1700≦FN/ε  (2)−2≦FN/EXP1≦2  (3)

In the above expressions, NA is the numerical aperture on the image sideof the tube lens 1, FN is the field number of the tube lens 1, c is theairy disk diameter (i.e., ε=1.22×λ/NA) with respect to d line (588 nm)of the tube lens 1, and EXP1 is the distance from the imaging plane tothe position of the exit pupil of the tube lens 1. EXP1 is expressed bya negative value when the position of the exit pupil is on the tube lens1 side with respect to the imaging plane, and is expressed by a positivevalue when the position of the exit pupil is on the side away from thetube lens 1 with respect to the imaging plane (that is, the sideopposite to the imaging lens with respect to the imaging plane). Notethat the position of the exit pupil of the tube lens 1 is the positionat which the principle ray passing through the center of the pupil ofthe objective 8 crosses with the optical axis. Also, the field numbercorresponds to twice the maximum image height. Note that when an opticalimage formed by the optical system of the microscope is picked up byusing the image pickup element 9 so as to conduct digital observation,it is desirable that the diagonal length of the image pickup element 9be roughly identical to the field number of the tube lens 1.

Condition expression (1) represents a condition for attaining sufficientresolution. Also, by NA not falling below the lower limit value ofcondition expression (1), it is possible to reduce sufficiently thediameter of the airy disk, making it possible to attain sufficientresolution. Also, a high NA leads to a large diameter of the ray passingthrough the tube lens 1, making it necessary to increase the diameter ofthe tube lens 1. By NA not exceeding the upper limit value of conditionexpression (1), it is possible to suppress the diameter of the tube lens1 to a level that permits production.

It is also possible to configure the tube lens 1 to satisfy thefollowing condition expression.0.04<NA≦0.08  (1-1)

It is also possible to configure the tube lens 1 to satisfy thefollowing condition expression.0.045≦NA≦0.08  (1-2)

Condition expression (2) represents a condition for attaining sufficientresolution and a wide field of view. By FN/ε not falling below the lowerlimit value of condition expression (2), it is possible to conductobservation of a sample and to acquire images with a wide field of viewand high resolution.

It is also possible to configure the tube lens 1 to satisfy thefollowing condition expression.1700≦FN/ε≦10000  (2-1)

Condition expression (3) represents a condition for maintainingtelecentricity. By FN/EXP1 being within the range of conditionexpression (3), the telecentricity of tube lens 1 is maintained.Thereby, it is possible to reduce influence of shading based on theangle characteristic of the image pickup element 9.

Note that an arbitrary combination from condition expression (1) throughcondition expression (18) may be applied to the tube lens. It is alsopossible to use only one of the upper limit value and the lower limitvalue of each of the expressions in order to limit the variables.

Hereinafter, specific explanations will be given for an example of theabove tube lens 1.

Example 1

FIG. 2 is a sectional view of a tube lens 10 according to the presentexample. The tube lens 10 exemplified in FIG. 2 is a microscope tubelens that forms an enlarged image of an object by being combined with aninfinity-corrected objective. The tube lens 10 includes, in the orderfrom the object side, a first lens group G1, having positive power, thatincludes a cemented lens CL1, a second lens group G2 having negativepower and a third lens group G3, having positive power as a whole, thatconsists of a plurality of lenses (L6 and L7) each having positivepower.

The first lens group G1 includes, in the order from the object side, abiconvex lens L1 and a cemented lens CL1 (first lens) including abiconvex lens L2 and a biconcave lens L3. The second lens group G2includes, in the order from the object side, a cemented lens CL2 (secondlens) including a biconcave lens L4 and a biconvex lens L5. The thirdlens group G3 includes, from the object side, a meniscus lens L6 (thirdlens) with positive power having the concave surface facing toward theobject side and a biconvex lens L7 (fourth lens). The first lens and thesecond lens respectively employ meniscus shapes.

In other words, the tube lens 10 including the three lens groupsrespectively having positive power, negative power and positive powercan identify the boundary between the first lens group G1 and the secondlens group G2 by utilizing the feature that the lens arranged closest tothe object in the second lens group G2 is the lens having the concavesurface facing toward the object side. Also, it is possible to identifythe boundary between the second lens group G2 and the third lens groupG3 by utilizing the feature that the third lens group G3 consists of aplurality of lenses having positive power.

Data of the tube lens 10 is as below. Note that the reference wavelengthis d line (587.56 nm). NA is the numerical aperture on the image side.The other symbols represent the same factors as in expressions (1)through (18) above.

NA=0.07, FN=30 mm,

ε=0.010248 mm, EXP1=1999.9952 mm

FL=180 mm, FLG1=142.1901 mm, FLG2=−81.4882 mm

D0=186.88 mm, D1=90.8598 mm, D2=162.2 mm

FLG3a=313.55 mm, FLG3b=151.876 mm

Ea=26.595 mm, Ec=22.093 mm

Lens data of the tube lens 10 is as below. Note that inf in the lensdata represents infinity (∞).

The tube lens 10 s r d nd vd s0 inf 162.2 s1 63.8523 9.3326 1.497 81.54s2 −1485.8995 3.1666 s3 39.1423 13.9864 1.497 81.54 s4 −145.3496 61.51633 64.14 s5 26.8639 20.2953 s6 −53.5928 8.0905 1.72047 34.71 s7110.3106 7.877 1.43875 94.93 s8 −130 14.8023 s9 −288.1082 6 1.5952267.74 s10 −114.1428 0.4703 s11 176.2945 6 1.85026 32.27 s12 −475.175490.8598442 s13 inf

In the above data, s represents the surface number, r represents thecurvature radius (mm), d represents the surface interval (mm), ndrepresents the refractive index with respect to d line and vd representsthe Abbe number. These symbols represent the same factors in thefollowing examples. Note that the surface represented by surface numbers0 is the surface of the position of the entrance pupil of the tube lens10 (position of the exit pupil of the objective), and the surfacerepresented by surface number s13 is the imaging plane. Also, surfaceinterval d0 for example represents the distance from the surfacerepresented by surface number s0 to the surface represented by surfacenumber s1.

As represented by expressions (1A) through (18A) below, the tube lens 10satisfies condition expressions (1) through (18), excluding conditionexpression (11). Note that expressions (1A) through (18A) correspond tocondition expressions (1) through (18), respectively. Also, expression(17A) is calculation that uses an example of the principal ray incidenton the position with the field number of 30.NA=0.07  (1A)FN/ε=2927  (2A)FN/EXP1=0.015  (3A)D2/FL=0.901  (4A)FLG1/FL=0.790  (5A)FLG2/FL=−0.453  (6A)D1/D0=0.486  (7A)ν1−νs=60.200  (8A)ν11−νs1=60.200  (9A)ν12−νs2=17.400  (10A)|RG2/RG1|=1.999  (11A)NdG2=1.720  (12A)νdG1=81.540  (13A)FLG3b/FLG3a=0.484  (14A)NdG3=1.850  (15A)νdG3=32.270  (16A)Ha/Hi=0.908  (17A)(Ea−Ec)/FL=0.025  (18A)

FIG. 3A through FIG. 3E are aberration diagrams of the tube lens 10exemplified in FIG. 2, and show aberrations on the imaging plane in acase when the parallel light beam is incident from the object side. FIG.3A is a spherical aberration diagram, FIG. 3B is a diagram of achromatic aberration of magnification, FIG. 3C is an astigmatismdiagram, FIG. 3D is a comma aberration diagram, and FIG. 3E is adistortion aberration diagram. Note that “M” in the figures representsMeridional component, and “S” represents Sagittal component.

Example 2

FIG. 4 is a sectional view of a tube lens 20 according to the presentexample. The tube lens 20 exemplified in FIG. 4 is a microscope tubelens that forms an enlarged image of an object by being combined with aninfinity-corrected objective. The tube lens 20 includes, in the orderfrom the object side, a first lens group G1, having positive power, thatincludes a cemented lens CL1, a second lens group G2 having negativepower and a third lens group G3, having positive power as a whole, thatconsists of a plurality of lenses (L6 and L7) each having positivepower. Note that the boundaries between the first lens group G1, thesecond lens group G2 and the third lens group G3 are identified in amanner similar to that in example 1.

The first lens group G1 includes, in the order from the object side, abiconvex lens L1 and a cemented lens CL1 (first lens) including ameniscus lens L2 with positive power having the convex surface facingtoward the object side and a meniscus lens L3 with negative power havingthe convex surface facing toward the object side. The second lens groupG2 includes, in the order from the object side, a cemented lens CL2(second lens) including a meniscus lens L4 with negative power havingthe concave surface facing toward the object side and a meniscus lens L5with positive power having the concave surface facing toward the objectside. The third lens group G3 includes, from the object side, a meniscuslens L6 (third lens) with positive power having the concave surfacefacing toward the object side and a biconvex lens L7 (fourth lens). Thefirst lens and the second lens respectively employ meniscus shapes.

Data of the tube lens 20 is as below. Note that the reference wavelengthis d line (587.56 nm).

NA=0.07, FN=30 mm,

ε=0.010248 mm, EXP1=1999.4602 mm

FL=180 mm, FLG1=160.3233 mm, FLG2=−11.2048 mm

D0=204.9437 mm, D1=107.891 mm, D2=162.2 mm

FLG3a=187.5652 mm, FLG3b=352.7182 mm

Ea=26.357 mm, Ec=23.38 mm

Lens data of the tube lens 20 is as below. Note that the surfacerepresented by surface number s0 is the surface of the position of theentrance pupil of the tube lens 20 (position of the exit pupil of theobjective), and the surface represented by surface number s13 is theimaging plane.

Tube lens 20 s r d nd vd s0 inf 162.2 s1 120.7472 13.1775 1.43875 94.93s2 −163.0581 9.7301 s3 41.4285 14.5537 1.497 81.54 s4 50 6.4656 1.5174252.43 s5 30.8345 10.7507 s6 −53.095 8.117 1.8 29.84 s7 −187.8877 7.86691.43875 94.93 s8 −124.275 14.613 s9 −252.1032 5.947 1.59522 67.74 s10−78.0578 0.6331 s11 351.1311 6 1.85026 32.27 s12 −2039.4189 107.0890528s13 inf

As represented by expressions (1B) through (18B) below, the tube lens 20satisfies condition expressions (1) through (18). Note that expressions(1B) through (18B) correspond to condition expressions (1) through (18),respectively. Also, expression (17B) is calculation that uses an exampleof the principal ray incident on the position with the field number of30.NA=0.07  (1B)FN/ε=2927  (2B)FN/EXP1=0.015  (3B)D2/FL=0.901  (4B)FLG1/FL=0.891  (5B)FLG2/FL=−0.062  (6B)D1/D0=0.526  (7B)ν1−νs=65.090  (8B)ν11−νs1=65.090  (9B)ν12−νs2=29.110  (10B)|RG2/RG1|=1.722  (11B)NdG2=1.800  (12B)νdG1=94.930  (13B)FLG3b/FLG3a=1.881  (14B)NdG3=1.850  (15B)νdG3=32.270  (16B)Ha/Hi=0.905  (17B)(Ea−Ec)/FL=0.017  (18B)

FIG. 5A through FIG. 5E are aberration diagrams of the tube lens 20exemplified in FIG. 4, and show aberrations on the imaging plane in acase when the parallel light beam is incident from the object side. Thetypes of these aberration diagrams are similar to those in example 1.

Example 3

FIG. 6 is a sectional view of a tube lens 30 according to the presentexample. The tube lens 30 exemplified in FIG. 6 is a microscope tubelens that forms an enlarged image of an object by being combined with aninfinity-corrected objective. The tube lens 30 includes, in the orderfrom the object side, a first lens group G1, having positive power, thatincludes a cemented lens CL1, a second lens group G2 having negativepower and a third lens group G3, having positive power as a whole, thatconsists of a plurality of lenses (L6 and L7) each having positivepower. Note that the boundaries between the first lens group G1, thesecond lens group G2 and the third lens group G3 are identified in amanner similar to that in example 1.

The first lens group G1 includes, in the order from the object side, ameniscus lens L1 with positive power having the convex surface facingtoward the object side, and a cemented lens CL1 (first lens) including ameniscus lens L2 with positive power having the convex surface facingtoward the object side and a meniscus lens L3 with negative power havingthe convex surface facing toward the object side. The second lens groupG2 includes, in the order from the object side, a cemented lens CL2(second lens) including a meniscus lens L4 with negative power havingthe concave surface facing toward the object side and a meniscus lens L5with positive power having the concave surface facing toward the objectside. The third lens group G3 includes, from the object side, a meniscuslens L6 (third lens) with positive power having the concave surfacefacing toward the object side and a meniscus lens L7 (fourth lens) withpositive power having the convex surface facing toward the object side.The first lens and the second lens respectively employ meniscus shapes.

Data of the tube lens 30 is as below. Note that the reference wavelengthis d line (587.56 nm).

NA=0.07, FN=30 mm,

ε=0.010248 mm, EXP1=1999.9441 mm

FL=180 mm, FLG1=256.5571 mm, FLG2=−254.0083 mm

D0=214.1499 mm, D1=110.7824 mm, D2=62.2 mm

FLG3a=291.7965 mm, FLG3b=341.1723 mm

Ea=17.925 mm, Ec=19.52 mm

Lens data of the tube lens 30 is as below. Note that the surfacerepresented by surface number s0 is the surface of the position of theentrance pupil of the tube lens 30 (position of the exit pupil of theobjective), and the surface represented by surface number s13 is theimaging plane.

Tube lens 30 s r d nd vd s0 inf 162.0987 s1 96.8617 8.2794 1.497 81.54s2 697.1144 2.9741 s3 41.59 14.0615 1.497 81.54 s4 283.4336 6 1.5163364.14 s5 31.9707 20.3068 s6 −62.9229 8.0051 1.74951 35.33 s7 −920.55057.7895 1.43875 94.93 s8 −64.3308 14.6329 s9 −283.2239 5.8629 1.5952267.74 s10 −108.4929 0.1694 s11 97.7459 15.286 1.85026 32.27 s12 136.8257110.782381 s13 inf

As represented by expressions (1C) through (18C) below, the tube lens 30satisfies condition expressions (1) through (18), excluding conditionexpression (10). Note that expressions (1C) through (18C) correspond tocondition expressions (1) through (18), respectively. Also, expression(17C) is calculation that uses an example of the principal ray incidenton the position with the field number of 30.NA=0.07  (1C)FN/ε=2927  (2C)FN/EXP1=0.015  (3C)D2/FL=0.346  (4C)FLG1/FL=1.425  (5C)FLG2/FL=−1.411  (6C)D1/D0=0.517  (7C)ν1−νs=59.600  (8C)ν11−νs1=59.600  (9C)ν12−νs2=17.400  (10C)|RG2/RG1|=1.968  (11C)NdG2=1.750  (12C)νdG1=91.930  (13C)FLG3b/FLG3a=1.169  (14C)NdG3=1.850  (15C)νdG3=32.270  (16C)Ha/Hi=0.346  (17C)(Ea−Ec)/FL=−0.009  (18C)

FIG. 7A through FIG. 7E are aberration diagrams of the tube lens 30exemplified in FIG. 6, and show aberrations on the imaging plane in acase when the parallel light beam is incident from the object side. Thetypes of these aberration diagrams are similar to those in example 1.

Example 4

FIG. 8 is a sectional view of a tube lens 40 according to the presentexample. The tube lens 40 exemplified in FIG. 8 is a microscope tubelens that forms an enlarged image of an object by being combined with aninfinity-corrected objective. The tube lens 40 includes, in the orderfrom the object side, a first lens group G1, having positive power, thatincludes a cemented lens CL1, a second lens group G2 having negativepower and a third lens group G3, having positive power as a whole, thatconsists of a plurality of lenses (L7 and L8) each having positivepower. Note that the boundaries between the first lens group G1, thesecond lens group G2 and the third lens group G3 are identified in amanner similar to that in example 1.

The first lens group G1 includes, in the order from the object side, abiconvex lens L1, a meniscus lens L2 with negative power having theconcave surface facing toward the object side, and a cemented lens CL1(first lens) including a meniscus lens L3 with positive power having theconvex surface facing toward the object side and a meniscus lens L4 withnegative power having the convex surface facing toward the object side.The second lens group G2 includes, in the order from the object side, acemented lens CL2 (second lens) including a biconcave lens L5 and abiconvex lens L6. The third lens group G3 includes, from the objectside, a meniscus lens L7 (third lens) with positive power having theconcave surface facing toward the object side and a meniscus lens L8(fourth lens) with positive power having the convex surface facingtoward the object side. The first lens and the second lens respectivelyemploy meniscus shapes.

Data of the tube lens 40 is as below. Note that the reference wavelengthis d line (587.56 nm).

NA=0.07, FN=30 mm,

ε=0.010248 mm, EXP1=1998.71 mm

FL=180 mm, FLG1=126.3122 mm, FLG2=−64.4325 mm

D0=197.606 mm, D1=111.3848 mm, D2=122.976 mm

FLG3a=92.0611 mm, FLG3b=193.5887 mm

Ea=25.037 mm, Ec=22.181 mm

Lens data of the tube lens 40 is as below. Note that the surfacerepresented by surface number s0 is the surface of the position of theentrance pupil of the tube lens 40 (position of the exit pupil of theobjective), and the surface represented by surface number s15 is theimaging plane.

Tube lens 40 s r d nd vd s0 inf 197.606 s1 47.7341 11.1697 1.497 81.54s2 −103.5787 3.3443 s3 −95.8974 4.5224 1.6516 58.55 s4 −4981.427 1.8525s5 84.0206 14.2254 1.497 81.54 s6 152.7909 6.2341 1.51633 64.14 s754.2161 21.4746 s8 −33.2007 7.938 1.83481 42.71 s9 141.3339 7.73031.43875 94.93 s10 −43.4424 14.4918 s11 −3446.5792 5.7559 1.59522 67.74s12 −101.9063 0.4627 s13 141.6394 12.183 1.85026 32.27 s14 978.566986.2206 s15 inf

As represented by expressions (1D) through (18D) below, the tube lens 40satisfies condition expressions (1) through (18), excluding conditionexpression (10). Note that expressions (1D) through (18D) correspond tocondition expressions (1) through (18), respectively. Also, expression(17D) is calculation that uses an example of the principal ray incidenton the position with the field number of 30.NA=0.07  (1D)FN/ε=2927  (2D)FN/EXP1=0.015  (3D)D2/FL=0.683  (4D)FLG1/FL=0.702  (5D)FLG2/FL=−0.358  (6D)D1/D0=0.564  (7D)ν1−νs=52.220  (8D)ν11−νs1=52.220  (9D)ν12−νs2=17.400  (10D)|RG2/RG1|=0.612  (11D)NdG2=1.835  (12D)νdG1=81.540  (13D)FLG3b/FLG3a=2.103  (14D)NdG3=1.850  (15D)νdG3=32.270  (16D)Ha/Hi=0.346  (17D)(Ea−Ec)/FL=0.016  (18D)

FIG. 9A through FIG. 9E are aberration diagrams of the tube lens 40exemplified in FIG. 8, and show aberrations on the imaging plane in acase when the parallel light beam is incident from the object side. Thetypes of these aberration diagrams are similar to those in example 1.

Example 5

FIG. 10 is a sectional view of a tube lens 50 according to the presentexample. The tube lens 50 exemplified in FIG. 10 is a microscope tubelens that forms an enlarged image of an object by being combined with aninfinity-corrected objective. The tube lens 50 includes, in the orderfrom the object side, a first lens group G1, having positive power, thatincludes a cemented lens CL1, a second lens group G2 having negativepower and a third lens group G3, having positive power as a whole, thatconsists of a plurality of lenses (L6 and L7) each having positivepower. Note that the boundaries between the first lens group G1, thesecond lens group G2 and the third lens group G3 are identified in amanner similar to that in example 1.

The first lens group G1 includes, in the order from the object side, abiconvex lens L1 and a cemented lens CL1 (first lens) including ameniscus lens L2 with positive power having the convex surface facingtoward the object side and a meniscus lens L3 with negative power havingthe convex surface facing toward the object side. The second lens groupG2 includes, in the order from the object side, a cemented lens CL2(second lens) including a meniscus lens L4 with negative power havingthe concave surface facing toward the object side and a meniscus lens L5with positive power having the concave surface facing toward the objectside. The third lens group G3 includes, from the object side, a meniscuslens L6 (third lens) with positive power having the concave surfacefacing toward the object side and a meniscus lens L7 (fourth lens) withpositive power having the convex surface facing toward the object side.The first lens and the second lens respectively employ meniscus shapes.

Data of the tube lens 50 is as below. Note that the reference wavelengthis d line (587.56 nm).

NA=0.07, FN=30 mm,

ε=0.010248 mm, EXP1=425.27 mm

FL=180 mm, FLG1=260.3535 mm, FLG2=−573.9846 mm

D0=226.03778 mm, D1=120.9627 mm, D2=222.2 mm

FLG3a=748.3104 mm, FLG3b=318.4502 mm

Ea=31.497 mm, Ec=28.291 mm

Lens data of the tube lens 50 is as below. Note that the surfacerepresented by surface number s0 is the surface of the position of theentrance pupil of the tube lens 50 (position of the exit pupil of theobjective), and the surface represented by surface number s13 is theimaging plane.

Tube lens 50 s r d nd vd s0 inf 222.2 s1 117.8297 9.5763 1.43875 94.93s2 −49640 3.6719 s3 59.5431 13.9171 1.618 63.33 s4 233.3093 6 1.5163364.14 s5 42.9641 21.3163 s6 −78.9796 8.1503 1.72047 34.71 s7 −2823.74497.9336 1.43875 94.93 s8 −65.9063 14.9811 s9 −282.8478 6.0675 1.497 81.54s10 −161.8049 0.7369 s11 152.7821 28.6118 1.83481 42.71 s12 328.6289105.075038 s13 inf

As represented by expressions (1E) through (18E) below, the tube lens 50satisfies condition expressions (1) through (18), excluding conditionexpression (10). Note that expressions (1E) through (18E) correspond tocondition expressions (1) through (18), respectively. Also, expression(17E) is calculation that uses an example of the principal ray incidenton the position with the field number of 30.NA=0.07  (1E)FN/ε=2927  (2E)FN/EXP1=0.071  (3E)D2/FL=1.234  (4E)FLG1/FL=1.446  (5E)FLG2/FL=−3.189  (6E)D1/D0=0.535  (7E)ν1−νs=60.220  (8E)ν11−νs1=60.220  (9E)ν12−νs2=0.810  (10E)|RG2/RG1|=1.838  (11E)NdG2=1.720  (12E)νdG1=94.930  (13E)FLG3b/FLG3a=0.426  (14E)NdG3=1.935  (15E)νdG3=42.710  (16E)Ha/Hi=1.244  (17E)(Ea−Ec)/FL=0.018  (18E)

FIG. 11A through FIG. 11E are aberration diagrams of the tube lens 50exemplified in FIG. 10, and show aberrations on the imaging plane in acase when the parallel light beam is incident from the object side. Thetypes of these aberration diagrams are similar to those in example 1.

Example 6

FIG. 12 is a sectional view of a tube lens 60 according to the presentexample. The tube lens 60 exemplified in FIG. 12 is a microscope tubelens that forms an enlarged image of an object by being combined with aninfinity-corrected objective. The tube lens 60 includes, in the orderfrom the object side, a first lens group G1, having positive power, thatincludes a cemented lens CL1, a second lens group G2 having negativepower and a third lens group G3, having positive power as a whole, thatconsists of a plurality of lenses (L6 and L7) each having positivepower. Note that the boundaries between the first lens group G1, thesecond lens group G2 and the third lens group G3 are identified in amanner similar to that in example 1.

The first lens group G1 includes, in the order from the object side, ameniscus lens (aspheric lens) L1 with positive power having the convexsurface facing toward the object side, and a cemented lens CL1 (firstlens) including a biconvex lens L2 and a biconcave lens L3. The secondlens group G2 includes, in the order from the object side, a cementedlens CL2 (second lens) including a meniscus lens L4 with negative powerhaving the concave surface facing toward the object side and a meniscuslens L5 with positive power having the concave surface facing toward theobject side. The third lens group G3 includes, from the object side, ameniscus lens L6 (third lens, aspheric lens) with positive power havingthe concave surface facing toward the object side and a meniscus lens L7(fourth lens, aspheric lens) with positive power having the convexsurface facing toward the object side. The first lens and the secondlens respectively employ meniscus shapes. The lenses L1, L6 and L7 areaspheric lenses, which are lenses whose both surfaces are aspheric.

Data of the tube lens 60 is as below. Note that the reference wavelengthis d line (587.56 nm).

NA=0.07, FN=30 mm,

Σ=0.010248 mm, EXP1=1978.7006 mm

FL=180 mm, FLG1=306.3939 mm, FLG2=−368.7981 mm

D0=215.8317 mm, D1=103.9357 mm, D2=162.2 mm

FLG3a=352.5076 mm, FLG3b=312.6439 mm

Ea=26.536 mm, Ec=25.611 mm

Lens data of the tube lens 60 is as below. Note that the surfacerepresented by surface number s0 is the surface of the position of theentrance pupil of the tube lens 60 (position of the exit pupil of theobjective), and the surface represented by surface number s13 is theimaging plane. Also, “*” beside surface numbers indicate that thesurfaces are aspheric.

Tube lens 60 s r d nd vd s0 inf 162.2 s1* 102.094 8.64 1.497 81.54 s2*430.3744 2.5322 s3 40.6782 14.02 1.497 81.54 s4 −1456.2435 5.99781.51633 64.14 s5 31.4159 19.7771 s6 −71.3563 8.012 1.72047 34.71 s7−352.2736 7.8017 1.43875 94.93 s8 −73.5208 14.6525 s9* −802.0276 5.87651.43875 94.93 s10* −159.9876 0.2926 s11* 102.985 16.3333 1.834 37.16s12* 167.4128 111.8960111 s13 inf

Aspheric data of the tube lens 60 is as below. In this example, theaspheric shape is expressed by the equation below, where Z is thecoordinate in the direction of the optical axis of the aspheric surface,Y is the coordinate in the direction orthogonal to the optical axis ofthe aspheric surface, K is the conic constant, r is the curvature radiusin a paraxial region of the aspheric surface, A2, A4, A6 and A8 are thesecond aspheric coefficient, the fourth aspheric coefficient, the sixthaspheric coefficient and the eighth aspheric coefficient, respectively,and E represents exponential of 10.

$Z = {\frac{Y^{2}}{r + {r\sqrt{1 - {\left( {K + 1} \right)\left( {Y\text{/}r} \right)^{2}}}}} + {A\; 2Y^{2}} + {A\; 4Y^{4}} + {A\; 6Y^{6}} + {A\; 8Y^{8}}}$

First surface s1 K = −2 A2 = 2.27E−04 A4 = 1.64E−06 A6 = 1.20E−09 A8 =1.02E−12 Second surface s2 K = −0.0716 A2 = 7.20E−05 A4 = 1.75E−06 A6 =1.41E−09 A8 = 1.29E−12 Ninth surface s9 K = −0.1061 A2 = 3.32E−04 A4 =3.98E−06 A6 = −6.42E−09 A8 = 4.60E−12 Tenth surface s10 K = −159.9876 A2= −3.9518E−04 A4 = 3.36E−06 A6 = −7.13E−09 A8 = 4.42E−12 Eleventhsurface s11 K = 102.985 A2 = −9.02E−05 A4 = 4.16E−08 A6 = 1.62E−09 A8 =−1.27E−12 Twelfth surface s12 K = 167.4128 A2 = 7.75E−05 A4 = 1.17E−07A6 = 3.29E−09 A8 = −2.40E−12

As represented by expressions (1F) through (18F) below, the tube lens 60satisfies condition expressions (1) through (18), excluding conditionexpression (10). Note that expressions (1F) through (18F) correspond tocondition expressions (1) through (18), respectively. Also, expression(17F) is calculation that uses an example of the principal ray incidenton the position with the field number of 30.NA=0.07  (1F)FN/ε=2927  (2F)FN/EXP1=0.015  (3F)D2/FL=0.901  (4F)FLG1/FL=1.702  (5F)FLG2/FL=−2.049  (6F)D1/D0=0.482  (7F)ν1−νs=60.220  (8F)ν11−νs1=60.220  (9F)ν12−νs2=17.400  (10F)|RG2/RG1|=2.271  (11F)NdG2=1.720  (12F)νdG1=81.540  (13F)FLG3b/FLG3a=0.887  (14F)NdG3=1.834  (15F)νdG3=37.160  (16F)Ha/Hi=0.907  (17F)(Ea−Ec)/FL=0.005  (18F)

FIG. 13A through FIG. 13E are aberration diagrams of the tube lens 60exemplified in FIG. 12, and show aberrations on the imaging plane in acase when the parallel light beam is incident from the object side. Thetypes of these aberration diagrams are similar to those in example 1.

Example 7

FIG. 14 is a sectional view of a tube lens 70 according to the presentexample. The tube lens 70 exemplified in FIG. 14 is a microscope tubelens that forms an enlarged image of an object by being combined with aninfinity-corrected objective. The tube lens 70 includes, in the orderfrom the object side, a first lens group G1, having positive power, thatincludes a cemented lens CL1, a second lens group G2 having negativepower and a third lens group G3, having positive power as a whole, thatconsists of a plurality of lenses (L6 and L7) each having positivepower. Note that the boundaries between the first lens group G1, thesecond lens group G2 and the third lens group G3 are identified in amanner similar to that in example 1.

The first lens group G1 includes, in the order from the object side, ameniscus lens L1 having the convex surface facing toward the objectside, and a cemented lens CL1 (first lens) including a biconvex lens L2and a biconcave lens L3. The second lens group G2 includes, in the orderfrom the object side, a cemented lens CL2 (second lens) including abiconcave lens L4 and a biconvex lens L5. The third lens group G3includes, from the object side, a meniscus lens L6 (third lens) withpositive power having the concave surface facing toward the object sideand a meniscus lens L7 (fourth lens) with positive power having theconvex surface facing toward the object side. The first lens and thesecond lens respectively employ meniscus shapes.

Data of the tube lens 70 is as below. Note that the reference wavelengthis d line (587.56 nm).

NA=0.07, FN=30 mm,

ε=0.010248 mm, EXP1=6143 mm

FL=180 mm, FLG1=503.2624 mm, FLG2=−81.4779 mm

D0=216.412 mm, D1=114.4765 mm, D2=162.2 mm

FLG3a=136.7465 mm, FLG3b=111.324 mm

Ea=26.47 mm, Ec=23.951 mm

Lens data of the tube lens 70 is as below. Note that the surfacerepresented by surface number s0 is the surface of the position of theentrance pupil of the tube lens 70 (position of the exit pupil of theobjective), and the surface represented by surface number s13 is theimaging plane.

Tube lens 70 s r d nd vd s0 inf 162.2 s1 89.9591 23.7981 1.497 81.54 s2158.2414 1 s3 50.3216 18.0151 1.497 81.54 s4 −95.0291 23.0142 1.5163364.14 s5 32.3459 10.4065 s6 −49.5543 6.4182 1.72047 34.71 s7 99.1751 5.51.43875 94.93 s8 −93.5214 1 s9 −132.5229 4.246 1.59522 67.74 s10 −51.0271 s11 65.2304 20.0785 1.85026 32.27 s12 180.1594 101.935897 s13 inf

As represented by expressions (1G) through (18G) below, the tube lens 70satisfies condition expressions (1) through (18), excluding conditionexpression (10). Note that expressions (1G) through (18G) correspond tocondition expressions (1) through (18), respectively. Also, expression(17G) is calculation that uses an example of the principal ray incidenton the position with the field number of 30.NA=0.07  (1G)FN/ε=2927  (2G)FN/EXP1=0.005  (3G)D2/FL=0.901  (4G)FLG1/FL=2.796  (5G)FLG2/FL=−0.453  (6G)D1/D0=0.529  (7G)ν1−νs=60.220  (8G)ν11−νs1=60.220  (9G)ν12−νs2=17.400  (10G)|RG2/RG1|=1.532  (11G)NdG2=1.720  (12G)νdG1=81.540  (13G)FLG3b/FLG3a=0.814  (14G)NdG3=1.850  (15G)νdG3=32.270  (16G)Ha/Hi=0.909  (17G)(Ea−Ec)/FL=0.014  (18G)

FIG. 15A through FIG. 15E are aberration diagrams of the tube lens 70exemplified in FIG. 14, and show aberrations on the imaging plane in acase when the parallel light beam is incident from the object side. Thetypes of these aberration diagrams are similar to those in example 1.

Example 8

FIG. 16 is a sectional view of a tube lens 80 according to the presentexample. The tube lens 80 exemplified in FIG. 16 is a microscope tubelens that forms an enlarged image of an object by being combined with aninfinity-corrected objective. The tube lens 80 includes, in the orderfrom the object side, a first lens group G1, having positive power, thatincludes a cemented lens CL1, a second lens group G2 having negativepower and a third lens group G3, having positive power as a whole, thatconsists of a plurality of lenses (L6 and L7) each having positivepower. Note that the boundaries between the first lens group G1, thesecond lens group G2 and the third lens group G3 are identified in amanner similar to that in example 1.

The first lens group G1 includes, in the order from the object side, abiconvex lens L1, and a cemented lens CL1 (first lens) including abiconvex lens L2 and a biconcave lens L3. The second lens group G2includes, in the order from the object side, a cemented lens CL2 (secondlens) including a biconcave lens L4 and a biconvex lens L5. The thirdlens group G3 includes, from the object side, a biconvex lens L6 (thirdlens) and a meniscus lens L7 (fourth lens) with positive power havingthe concave surface facing toward the object side. The first lens andthe second lens respectively employ meniscus shapes.

Data of the tube lens 80 is as below. Note that the reference wavelengthis d line (587.56 nm).

NA=0.07, FN=30 mm,

ε=0.010248 mm, EXP1=108.99 mm

FL=180 mm, FLG1=177.491 mm, FLG2=−79.0664 mm

D0=268.9759 mm, D1=214.0886 mm, D2=162.2 mm

FLG3a=102.58 mm, FLG3b=221.0191 mm

Ea=26.387 mm, Ec=27.006 mm

Lens data of the tube lens 80 is as below. Note that the surfacerepresented by surface number s0 is the surface of the position of theentrance pupil of the tube lens 80 (position of the exit pupil of theobjective), and the surface represented by surface number s13 is theimaging plane.

Tube lens 80 s r d nd vd s0 inf 162.2 s1 108.373 44.1565 1.497 81.54 s2−420.1585 17.4045 s3 37.7218 16.3353 1.497 81.54 s4 −147.0718 7.28941.51633 64.14 s5 26.8628 24.5481 s6 −39.8089 12.9467 1.72047 34.71 s777.0609 27.5053 1.43875 94.93 s8 −71.3383 1 s9 196.7487 10.569 1.696855.53 s10 −109.7848 8.7513 s11 −700 43.5824 1.85026 32.27 s12 −152.746654.887315 s13 inf

As represented by expressions (1H) through (18H) below, the tube lens 80satisfies condition expressions (1) through (18), excluding conditionexpression (10). Note that expressions (1H) through (18H) correspond tocondition expressions (1) through (18), respectively. Also, expression(17H) is calculation that uses an example of the principal ray incidenton the position with the field number of 30.NA=0.07  (1H)FN/ε=2927  (2H)FN/EXP1=0.275  (3H)D2/FL=0.901  (4H)FLG1/FL=0.986  (5H)FLG2/FL=−0.439  (6H)D1/D0=0.796  (7H)ν1−νs=60.220  (8H)ν11−νs1=60.220  (9H)ν12−νs2=17.400  (10H)|RG2/RG1|=1.482  (11H)NdG2=1.720  (12H)νdG1=81.540  (13H)FLG3b/FLG3a=2.155  (14H)NdG3=1.850  (15H)νdG3=32.270  (16H)Ha/Hi=0.906  (17H)(Ea−Ec)/FL=−0.003  (18H)

FIG. 17A through FIG. 17E are aberration diagrams of the tube lens 80exemplified in FIG. 16, and show aberrations on the imaging plane in acase when the parallel light beam is incident from the object side. Thetypes of these aberration diagrams are similar to those in example 1.

Example 9

FIG. 18 is a sectional view of a tube lens 90 according to the presentexample. The tube lens 90 exemplified in FIG. 18 is a microscope tubelens that forms an enlarged image of an object by being combined with aninfinity-corrected objective. The tube lens 90 includes, in the orderfrom the object side, a first lens group G1, having positive power, thatincludes a cemented lens CL1, a second lens group G2 having negativepower and a third lens group G3, having positive power as a whole, thatconsists of a plurality of lenses (L6 and L7) each having positivepower. Note that the boundaries between the first lens group G1, thesecond lens group G2 and the third lens group G3 are identified in amanner similar to that in example 1.

The first lens group G1 includes, in the order from the object side, abiconvex lens L1, and a cemented lens CL1 (first lens) including abiconvex lens L2 and a biconcave lens L3. The second lens group G2includes, in the order from the object side, a cemented lens CL2 (secondlens) including a biconcave lens L4 and a biconvex lens L5. The thirdlens group G3 includes, from the object side, a biconvex lens L6 (thirdlens) and a meniscus lens L7 (fourth lens) with positive power havingthe convex surface facing toward the object side. The first lens and thesecond lens respectively employ meniscus shapes.

Data of the tube lens 90 is as below. Note that the reference wavelengthis d line (587.56 nm).

NA=0.07, FN=30 mm,

ε=0.010248 mm, EXP1=358.0679 mm

FL=180 mm, FLG1=176.1303 mm, FLG2=−78.0404 mm

D0=248.8623 mm, D1=169.271 mm, D2=82.2 mm

FLG3a=100.5002 mm, FLG3b=452.1581 mm

Ea=17.918 mm, Ec=24.447 mm

Lens data of the tube lens 90 is as below. Note that the surfacerepresented by surface number s0 is the surface of the position of theentrance pupil of the tube lens 90 (position of the exit pupil of theobjective), and the surface represented by surface number s13 is theimaging plane.

Tube lens 90 s r d nd vd s0 inf 82.2 s1 109.277 30 1.497 81.54 s2−257.1081 3.8857 s3 48.498 25.5599 1.497 81.54 s4 −52.4895 13.35631.51633 64.14 s5 26.6665 10.0652 s6 −53.2581 35 1.72047 34.71 s7 76.785931.0012 1.43875 94.93 s8 −135.6727 4.8645 s9 150.0768 10.8424 1.5952267.74 s10 −96.7851 1 s11 115.4106 3.696 1.85026 32.27 s12 162.491279.5913 s13 inf

As represented by expressions (1I) through (18I) below, the tube lens 90satisfies condition expressions (1) through (18), excluding conditionexpressions (10) and (18). Note that expressions (1I) through (18I)correspond to condition expressions (1) through (18), respectively.Also, expression (17I) is calculation that uses an example of theprincipal ray incident on the position with the field number of 30.NA=0.07  (1I)FN/ε=2927  (2I)FN/EXP1=0.084  (3I)D2/FL=0.457  (4I)FLG1/FL=0.979  (5I)FLG2/FL=−0.434  (6I)D1/D0=0.680  (7I)ν1−νs=60.220  (8I)ν11−νs1=60.220  (9I)ν12−νs2=17.400  (10I)|RG2/RG1|=1.997  (11I)NdG2=1.721  (12I)νdG1=81.540  (13I)FLG3b/FLG3a=4.499  (14I)NdG3=1.850  (15I)νdG3=32.270  (16I)Ha/Hi=0.459  (17I)(Ea−Ec)/FL=−0.036  (18I)

FIG. 19A through FIG. 19E are aberration diagrams of the tube lens 90exemplified in FIG. 18, and show aberrations on the imaging plane in acase when the parallel light beam is incident from the object side. Thetypes of these aberration diagrams are similar to those in example 1.

What is claimed is:
 1. A tube lens that is for a microscope and thatforms an enlarged image of an object by being combined with aninfinity-corrected objective, the tube lens comprising in an order froman object side: a first lens group that has positive power and thatincludes a cemented lens; a second lens group that has negative power;and a third lens group that consists of a plurality of lenses eachhaving positive power and that has positive power as a whole, wherein afollowing condition expression is satisfied, where FL is a focal lengthof the tube lens and D2 is a distance from a lens surface that isclosest to the object in the tube lens to a position of an entrancepupil of the tube lens:0.3<D2/FL<1.3.
 2. The tube lens according to claim 1, wherein followingcondition expressions are satisfied, where FLG1 is a focal length of thefirst lens group, FLG2 is a focal length of the second lens group, D0 isa distance from a lens surface that is closest to the object in the tubelens to an imaging plane, on which the enlarged image is formed, and D1is a distance from a lens surface that is closest to the object in thetube lens to a lens surface that is closest to the imaging plane in thetube lens:0.3<FLG1/FL<3;−4<FLG2/FL<−0.05; and0.3<D1/D0<0.8.
 3. The tube lens according to claim 1, wherein the firstlens group includes a first lens having a concave surface facing towardan image side, and the second lens group includes a second lens having aconcave surface facing toward an object side.
 4. The tube lens accordingto claim 3, wherein the first lens is a meniscus lens that is arrangedclosest to an image plane in the first lens group, that has a concavesurface facing toward an image side, and that includes a single lens ora cemented lens, and the second lens is a meniscus lens that is arrangedclosest to the object in the second lens group, that has a concavesurface facing toward an object side, and that includes a single lens ora cemented lens.
 5. The tube lens according to claim 4, wherein thefirst lens and the second lens include at least four lenses in total,and at least one of the first lens and the second lens includes acemented lens.
 6. The tube lens according to claim 5, wherein the firstlens and the second lens are cemented lenses, respectively.
 7. The tubelens according to claim 4, wherein at least one of the first lens andthe second lens is a cemented lens including a lens having positivepower and a lens having negative power, and a following conditionexpression is satisfied, where ν1 is a highest Abbe number among Abbenumbers of lenses with positive power included in the cemented lens andνs is a lowest Abbe number among Abbe numbers of lenses with negativepower included in the cemented lens:30<ν1−νs.
 8. The tube lens according to claim 6, wherein the first lensand the second lens are cemented lenses each including a lens havingpositive power and a lens having negative power, and following conditionexpressions are satisfied, where ν11 is a highest Abbe number among Abbenumbers of lenses with positive power included in a first cemented lens,which is one of the first lens and the second lens, νs1 is a lowest Abbenumber among Abbe numbers of lenses with negative power included in thefirst cemented lens, ν12 is a highest Abbe number among Abbe numbers oflenses with positive power included in a second cemented lens, which isthe other one of the first lens and the second lens, and νs2 is a lowestAbbe number among Abbe numbers of lenses with negative power included inthe second cemented lens:30<ν11−νs1; and25<ν12−νs2.
 9. The tube lens according to claim 3, wherein followingcondition expressions are satisfied, where RG1 is a curvature radius ofthe concave surface of the first lens facing toward an image side, RG2is a curvature radius of the concave surface of the second lens facingtoward an object side, NdG2 is a refractive index with respect to d lineof the second lens or a refractive index with respect to d line of alens closest to the object among lenses constituting the second lens,and νdG1 is a highest Abbe number among Abbe numbers of lenses withpositive power included in the first lens group:0<|RG2/RG1|<3;1.5<NdG2; and70<νdG1.
 10. The tube lens according to claim 1, wherein the third lensgroup includes a meniscus lens with positive power having a concavesurface facing toward an object side.
 11. The tube lens according toclaim 1, wherein the third lens group comprises: a third lens withpositive power that is arranged closest to the object; and a fourth lenswith positive power that is arranged closest to an image plane, and afollowing condition expression is satisfied, where FLG3a is a focallength of the third lens and FLG3b is a focal length of the fourth lens:0.2<FLG3b/FLG3a<5.
 12. The tube lens according to claim 1, whereinfollowing condition expressions are satisfied, where NdG3 is a highestrefractive index among refractive indexes of lenses included in thethird lens group and νdG3 is a lowest Abbe number among Abbe numbers oflenses included in the third lens group:1.7<NdG3; andνdG3<45.
 13. The tube lens according to claim 1, wherein a followingcondition expression is satisfied, where Ha is a height of an off-axisprincipal ray when the off-axis principal ray is incident on a lenssurface closest to the object in the tube lens and Hi is a height of theoff-axis principal ray when the off-axis principal ray is incident on animaging plane:0.3<Ha/Hi<1.
 14. The tube lens according to claim 1, wherein a followingcondition expression is satisfied, where Ea is a maximum effectivediameter of a lens included in the first lens group and Ec is a maximumeffective diameter of a lens included in the third lens group:−0.04<(Ea−Ec)/FL<0.03.
 15. A microscope apparatus comprising a tube lensaccording to claim
 1. 16. A microscope apparatus according to claim 15,further comprising an image pickup element that picks up the enlargedimage of the object formed by the tube lens, wherein a followingcondition expression is satisfied, where L is a pixel size of the imagepickup element:1 μm≦L≦17 μm.
 17. An image pickup optical system comprising: aninfinity-corrected objective; and a microscope tube lens, wherein themicroscope tube lens includes in an order from an object side: a firstlens group that has positive power and that includes a cemented lens; asecond lens group that has negative power; and a third lens group thatconsists of a plurality of lenses each having positive power and thathas positive power as a whole, and a following condition expression issatisfied, where FL is a focal length of the tube lens and D2 is adistance from a lens surface that is closest to an object in the tubelens to a position of an entrance pupil of the tube lens:0.3<D2/FL<1.3.